The present invention relates to test equipment and measurements using test equipment. More particularly, the invention relates to measurement of phase noise of a signal under test.
Phase noise is noise that manifests itself as unwanted, usually random, fluctuations in a relative phase of a signal. Many modem systems, such as phased array radars, the global positioning satellite system (GPS) and communications systems that employ one or more forms of phase modulation, depend on precise, accurate knowledge of signal phase for their operation. Phase noise directly interferes with the operation of these and related systems. Therefore, the measurement of phase noise is an important topic in the field of test and measurement. In particular, an important objective of phase noise measurement is to obtain an accurate measurement of phase noise for a given signal under test (SUT).
There are a number of approaches known in the art for measuring phase noise, each method having its own advantages and disadvantages. For example, phase noise can be measured using a frequency discriminator or using a delay line and mixer as a frequency comparator. Another approach uses two sources and a phase comparator. Yet another approach measures phase noise using a heterodyne frequency measurement technique. However, among all of the approaches to phase noise measurement, perhaps the most popular, practical, and cost-effective approach employs a spectrum analyzer.
A spectrum analyzer is a device or system that measures the power spectral density (PSD) of a signal or one of several closely related signal parameters. For the purposes of discussion herein, the PSD of a signal can be thought of as a measurement of signal power in a selected bandwidth as a function of frequency. Typically, the spectrum analyzer displays measured PSD in the form of a graph. Phase noise measurements can be extracted or computed from measured PSD of the SUT. Most spectrum analyzers, especially those used for high frequency radio frequency (RF), are implemented as heterodyne receivers that frequency shift or frequency-convert the signal prior to detecting and measuring the power of the signal. A typical spectrum analyzer may employ three or four stages of frequency conversion prior to a signal power measurement. One or more of these frequency conversion stages generally utilizes a swept or stepped frequency local oscillator (LO) to provide frequency scanning or tuning.
Unfortunately, phase noise measurements obtained using spectrum analyzers inevitably contain errors that distort and in some cases, even obscure the true phase noise of the signal under test. In practice, the LOs, which are used in the various frequency conversions and in aspects of detecting and measuring signal power in the spectrum analyzer, introduce or add phase noise to the SUT being measured. The added phase noise is typically independent of the signal being measured and is solely due to the operations performed by the spectrum analyzer on the SUT. For example, a major source of added phase noise in the spectrum analyzer is phase noise of a first LO used in a first frequency conversion stage of the spectrum analyzer. The end result is that the magnitude of the phase noise, as measured by the spectrum analyzer, is generally greater than the true or actual phase noise of the SUT due to this added phase noise.
A conventional approach to mitigating the added phase noise effects of the spectrum analyzer used to measure phase noise of the SUT normally involves simply using a better spectrum analyzer. In simple terms, a better spectrum analyzer is one that has lower added phase noise. The lower the added phase noise, the less that added phase noise corrupts the measurements of phase noise of the SUT. Lower added phase noise in a spectrum analyzer is typically achieved by using cleaner, more stable LOs. This is especially true for the LO used in the first frequency conversion stage or stages. In addition, the sensitivity of the signal detection circuitry of the spectrum analyzer must be higher to avoid masking of the measured phase noise by internal thermal noise of the spectrum analyzer. Thus, a better spectrum analyzer, having lower added phase noise, is the result of using better, higher quality components to construct the spectrum analyzer.
However, improving the added phase noise performance in a spectrum analyzer (i.e. reducing added phase noise) typically comes at a price. Even moderate improvements in phase noise performance from one model of spectrum analyzer to another can often result in significant increases in unit price. The increased unit price is due in large part to increased costs of better, higher performance LOs and/or higher sensitivity detection circuitry necessary to implement spectrum analyzers with lower added phase noise. Much the same thing can be said for the other phase noise measurement approaches known in the art. Increased phase noise measurement accuracy using better measurement devices can become very, sometimes even prohibitively, expensive.
Accordingly, it would be advantageous to have a more economical approach to obtaining accurate phase noise measurements than simply using a better, more expensive spectrum analyzer. Moreover, it would be advantageous if such an approach could improve accuracy of phase noise measurements produced by virtually any spectrum analyzer, even ones with lower added phase noise. Such an approach would solve a long-standing need in the area of economical phase noise measurement using spectrum analyzers.
The present invention is a phase noise measurement module (PNMM) and method of measuring phase noise that, when used in conjunction with a spectrum analyzer, can improve the accuracy of phase noise measurements of a signal under test (SUT). According to the present invention, a first radio frequency (RF)-to-intermediate frequency (IF) conversion stage in the spectrum analyzer is bypassed during a phase noise measurement of a signal under test (SUT). Instead, the SUT is frequency converted directly to an IF signal having a center frequency that corresponds to either a second IF frequency or a third IF frequency of the spectrum analyzer. In addition, the direct conversion of the present invention generates a tunable direct conversion LO signal from a tunable local oscillator (LO) in the first conversion stage of the spectrum analyzer. The direct conversion LO signal is a frequency divided LO signal derived through the use of a selectable frequency divider. The frequency divided LO signal is mixed with the SUT in a frequency converter of the PNMM to produce the IF signal. The combination of direct conversion of the SUT and selectable frequency division of the LO signal of the present invention results in less added phase noise to the SUT and therefore, more accurate phase noise measurements, than can be achieved with the spectrum analyzer alone.
In one aspect of the invention, a PNMM or direct-conversion apparatus for use in conjunction with a spectrum analyzer having multiple frequency conversion stages is provided. The PNMM facilitates accurate phase noise measurements of an SUT performed using the multi-stage spectrum analyzer. Preferably, the spectrum analyzer used with the present invention has at least three IF frequency conversion stages. The PNMM has an RF input port, an LO input port and an IF output port and comprises an RF to IF frequency converter having an RF input port, an LO input port and an IF output port. The RF input port of the frequency converter is connected to the RF input port of the PNMM apparatus and the IF output port is connected to the IF output port of the PNMM. The PNMM further comprises a selectable frequency divider having an input port that is connected to the LO input port of the PNMM apparatus, and an output port that is connected to the LO input port of the frequency converter. The selectable frequency divider has a selectable division factor.
In some embodiments, the PNMM may further comprise a switch having an input port connected to the IF output port of the frequency converter and an output port connected to the PNMM IF output port. The switch directs an IF signal from the frequency converter to either the second conversion stage or the third conversion stage of the spectrum analyzer via the PNMM IF output port.
An SUT applied to the RF input port of the PNMM is filtered by an optional lowpass filter and passes on to the RF input port of the frequency converter. A tunable LO signal produced by the spectrum analyzer in the first conversion stage is applied to the LO input port of the PNMM apparatus and therethrough to the LO input port of the selectable frequency divider. The selectable frequency divider produces a frequency divided LO signal from the spectrum analyzer""s first conversion stage tunable LO. The divided LO signal is mixed with the SUT in the frequency converter to produce an IF signal. The IF signal has a frequency that is centered either at a second IF frequency of the spectrum analyzer or a third IF frequency of the spectrum analyzer depending on the selected division factor of the selectable divider and a center frequency of the SUT. The IF signal passes to the IF output port of the PNMM apparatus, where it is routed to either the second or third frequency conversion stage of the spectrum analyzer, depending on the IF frequency of the produced IF signal.
In another aspect of the present invention, a system for measuring phase noise is provided. The system of the present invention comprises the PNMM described above, and further comprises a spectrum analyzer having multiple conversion stages, an RF input, an LO output connected to a second output of a tunable LO in a first conversion stage of the spectrum analyzer, and an additional IF input in each of a second and a third conversion stages. The LO output of the spectrum analyzer is connected to the PNMM at the PNMM LO input port. The IF input of the second and third conversion stages are connected to the PNMM at the PNMM IF output port. The PNMM bypasses the first conversion stage of the spectrum analyzer and directly converts a SUT into an IF signal having a frequency centered at either the second or third IF frequency of the spectrum analyzer.
In still another aspect of the present invention, a method of measuring phase noise using a spectrum analyzer that has multiple frequency conversion stages is provided. The method of measuring phase noise comprises directly converting an input signal under test at an input of the spectrum analyzer to an IF signal having either a second or a third IF frequency. The IF signal is routed to a second or a third frequency conversion stage of the spectrum analyzer, depending on the IF frequency of the IF signal, before phase noise is measured.
The SUT is directly converted comprising frequency dividing a tunable first LO signal from a first conversion stage of the spectrum analyzer, and mixing the divided tunable LO signal with the SUT to produce the IF signal. Either the second or the third frequency conversion stage is selected and the IF signal is applied to the selected second or third frequency conversion stage of the spectrum analyzer. The method of measuring phase noise optionally further comprises identifying and locating any frequency components of the SUT that lie at image frequencies of the direct conversion. Phase noise is measured by processing the directly converted IF signal in a conventional manner using the spectrum analyzer.
The present invention facilitates obtaining more accurate phase noise measurements than the conventional approaches that employ the spectrum analyzer alone. The present invention improves phase noise measurement accuracy primarily by reducing the phase noise added to the SUT by the multiple frequency conversions of the SUT in the spectrum analyzer. To begin with, bypassing the first two RF to IF conversion stages in the spectrum analyzer essentially eliminates phase noise added to the SUT by the first and second LOs of the spectrum analyzer. What phase noise is added to the SUT by the tunable direct conversion LO signal of the invention is minimized by frequency dividing the LO signal from the tunable LO of the spectrum analyzer.
Advantageously, the present invention can improve the accuracy of phase noise measurements obtained using almost any spectrum analyzer provided that access to the tunable first LO, second IF stage, and third IF stage of the spectrum analyzer is provided. Thus, even measurements obtained with high performance spectrum analyzers can be improved according to the present invention. Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings.